This invention relates generally to radar altimeters, and more specifically to a radar altimeter with a forward looking capability.
The proper navigation of an aircraft in all phases of its flight is based, to a large extent, upon the ability to determine the terrain over which it is passing, and further based on the ability to determine a position of the aircraft. In this regard, aircraft instrumentation, sensors, radar systems, and specifically, radar altimeters are used in combination with electronic terrain maps. The electronic terrain maps (sometimes referred to as digital elevation maps or DEMs) provide the height (elevation) of objects on the map, and together with the radar altimeter, aid in the flight and the planning of a flight path for the aircraft.
As such, radar altimeters are commonly implemented within aircraft. A radar altimeter typically includes a transmitter for applying pulses of electromagnetic energy at regular intervals to an antenna which then radiates the energy, in the form of a transmit beam, towards the earth's surface. A transmit beam from a radar is sometimes said to “illuminate” an area which reflects the transmit beam.
The radar altimeter further includes a signal receiver which receives return pulses, sometimes referred to as an echo or a return signal. Return pulses are received at an receive antenna, and constitute transmit beams that have been reflected from the earth's surface. It is known that some radar altimeters utilize the same antenna for both transmitting and receiving. A closed loop servo tracker for measuring a time interval between the transmitted pulse and its associated return pulse also forms a part of the radar altimeter. The time interval between the transmit pulse and the return pulse is directly related to the altitude of the aircraft.
However, problems still exist with flights into certain terrain. For example, aircraft, especially helicopters, are sometimes required to fly at very low altitudes. Flying at such low altitudes increases the probability that certain terrain features are in front of the aircraft, in the flight path, rather than safely below the aircraft, as is the case at normal flight altitudes.
Radar altimeters are generally incapable of detecting objects that are in a flight path. Examples of such objects include, for example, tall buildings, or the side of a cliff. While an aircraft equipped with a radar altimeter can determine an altitude, the aircraft is not able to determine the presence of objects in front of the aircraft if not equipped with, for example, a costly scanning laser radar. Problems also exist even when the scanning laser radar is implemented within an aircraft since they are sometimes rendered ineffective when encountering one or more of rain, fog, and smoke.
As described above, certain helicopter missions are flown at a very low altitude, for example, 20 to 100 feet. Such nap of the earth flights (e.g., low level contoured flights over the earth surface), may include flying around certain obstacles, to maintain as low a profile as possible in order to minimize detection by enemy forces. Medical emergency response missions also often require low altitude operations. Electronic digital elevation maps (DEMs) have been integrated with navigation systems, for example, radar altimeters and inertial measurement units, to provide a look ahead capability, based on the data in the DEM, that allows a pilot to see ahead through the weather, on a heads up or cockpit display. Such integrated systems therefore provide a display of obstacles recorded within the DEM based on a position as determined by the navigation systems. However, DEM data can be inaccurate, for example, due to an addition of new manmade structures after the DEM data was collected. These possible DEM inaccuracies have resulted in a lack of confidence to safely fly at the desired low altitudes during low visibility conditions.